This invention relates to the detection of temperature transient conditions in superconducting magnets and, more particularly, to the use of fiber optics to detect temperature transient detection in superconducting magnets.
It is a characteristic of superconducting magnets that the superconducting wire forming the magnet coil may undergo a spontaneous quench condition, i.e., change from a superconducting state to a normal state. A quench condition is a very localized condition that results in a localized hot spot from the sudden increase in wire resistance and concomitant resistance heating. This localized heating can produce temperatures in excess of 400-800 K. that cause failure in the wire insulation and degradation of the superconductor critical current. Thus, it is highly desirable to detect a temperature transient condition and apply rapid heating to the entire magnet in a time scale commensurate with the onset of the temperature transient, in the order of 100 msec.
Fiber optic-based systems have been used for temperature transient detection. Fiber optic-based systems have many advantages over conventional voltage monitor systems for quench detection, e.g., very low thermal conductivity, high resistance to EMI interference (with resulting low false trigger rates), high radiation resistance, reduced mechanical complexity, high system reliability due to a reduced number of components, and efficient multiplexing capabilities. O. Tsukamoto et al., "Detection of Temperature Rise at 4.2 K. by Using a Dual-Core Optical Fiber--An Optical Method to Detect a Quency of a Superconducting Magnet," 31 Advances in Cryogenic Engineering, Plenum House, New York (1986), pp.1269, teaches an optical fiber system using a dual-core fiber. A temperature rise of 1.0 K. at 4.2 K. is detected, equivalent to a quench in a superconducting magnet. As taught by Tsukamoto, a laser light is split and input to outer and core fibers. A temperature transient condition causes a change in the optical length of the outer fiber, primarily from a change in the refractive index, wherein interference fringes are formed when the light signals from the inner and outer fibers are combined at the output of the dual-core optical fiber. The dual-core fibers are, however, somewhat difficult to incorporate into a magnet winding. The dual core fiber also has a sensitivity limitation since both fibers are adjacent any temperature transient event so both the outer and inner fiber are responding to the same temperature change, albeit at different rates. Further, dual-core fibers require doped fibers that are inherently radiation sensitive and would not be useable in some important applications, e.g., high energy particle accelerators, such as the superconducting supercollider, and superconducting magnets for fusion application, e.g. ITER.
Tsukamoto does teach that two single-mode fibers may be used in a Mach-Zender arrangement to improve on the sensitivity of the dual fiber arrangement. Then, one fiber provides a reference signal and the other fiber a temperature signal. Tsukamoto notes, however, that there are difficulties in placing this arrangement in a cryogenic region and in making the system compact enough to be a quench detector. There is no teaching about placing the reference and temperature sensor fibers on superconducting magnets, but the statement on placement difficulties and the teachings on the dual core fibers infer that the two fibers are placed on the same magnet to subject both fibers to the same environment to minimize noise
Accordingly, it is an object of the present invention to provide a single-core optical fiber system for detecting temperature transients in superconducting magnets, wherein only a single fiber is incorporated in a magnet to limit the space required to incorporate the fiber sensor.
It is another object of the present invention to provide a high sensitivity system using separated reference and sensor fibers for rapid detection of temperature transients in superconducting magnets.
It is a further object of the present invention to detect temperature transients in superconducting magnets using undoped optical fibers for improved transmission stability in a high radiation environment.
One other object of the present invention is to provide a Mach-Zender type arrangement with improved noise rejection.
An additional object of the present invention is to reduce the number of detector fibers required in large systems.
Still another object of the present invention is to reduce the number of false quench alarms arising from a large array of superconducting magnets.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.